1. Field of the Invention
The present invention relates to a zoom lens control apparatus, a zoom lens system, and an image-taking system which uses the zoom lens control apparatus.
2.Description of Related Art
When a zoom lens is mounted on a television camera or a video camera, it is necessary to adjust a flange focal distance to match the position of an image plane to the position of the image-pickup device (such as a CCD) of the camera. The flange focal distance means a distance from a plane of the camera (a flange plane) on which the zoom lens is mounted, to the image plane of the zoom lens.
If the flange focal distance deviates from an appropriate value, disadvantages occur such as an image blur caused from zooming even when an image of a stationary object is taken. Especially in a zoom lens apparatus of a rear focus type which achieves focusing by a lens located on the side of a stop (diaphragm) closer to an image plane, a data table which includes a moving track of the rear focus lens on an optical axis (a zoom tracking curve) is previously stored in a memory for each object distance such that an in-focus state can be maintained by moving the rear focus lens in electric synchronization with zooming.
In such a zoom lens of the rear focus type, the flange focal distance needs to be adjusted to match a reference position in the data table to the position of the rear focus lens in order to allow accurate focusing.
Japanese Patent Application Laid-Open No. H7-154667 (No. 1995-154667) has proposed a method of adjusting a flange focal distance by finding an inflection point of a zoom tracking curve as described above and then determining the positions of the wide-angle end and the telephoto end.
Japanese Patent Application Laid-Open No. H12-121911 (No. 2000-121911) has proposed a method of adjusting a flange focal distance in a lens system of a type with no inflection point on a zoom tracking curve as described above. Specifically, when a lens for varying the magnification is located at the designed wide-angle end and the designed telephoto end, focus positions are found at the respective zoom positions through automatic focusing control to determine actual adjustment values in a zoom direction and a focus direction from a previously prepared table.
Japanese Patent Application Laid-Open No. H11-127376 (No. 1999-127376) has proposed a method of automating adjustment of a flange focal distance in a zoom lens apparatus of a front focus type in which a focus lens and a back focal distance adjusting lens can be electrically driven to perform automatic focusing control.
In the flange focal distance adjusting methods proposed in Japanese Patent Application Laid-Open No. H7-154667 and No. 2000-121911, however, an appropriate object must be disposed at a specific distance in adjusting the flange focal distance. This places a restriction on the flange focal distance adjustment at the site of image-taking, and in addition, the accuracy of the flange focal distance adjustment may be reduced if the actual object distance deviates from the specific distance.
In the flange focal distance adjusting method proposed in Japanese Patent Application Laid-Open No. H11-127376, each of the focus lens and the back focal distance adjusting lens needs to be electrically driven. In addition, it is necessary to construct a system which allows automatic focus detection for each of the lenses, so that the resulting system has a complicated structure.